Hydraulic damping devices are used in a variety of applications to damp pressure fluctuations and forces in position controlled systems. One area where such devices are used is that of hydraulically powered actuators in aircraft systems. In an aircraft system, hydraulically powered actuators are used in a wide range of applications. One common area of application is in the actuation of control surfaces of the aircraft, for example in the actuation of nose wheel steering, elevators, rudders, ailerons, and so on. Typically the actuator must be able to extend and retract in order to allow deployment and retraction of the control system. To this end, the actuator will normally have an extension chamber into which hydraulic fluid is admitted to extend the actuator, and a retraction chamber into which hydraulic fluid is admitted to retract the actuator, i.e. move it in an opposite direction from extension. A series of valves selectively connect the extension and retraction chambers to a source of hydraulic fluid to effect the appropriate movement of the actuator.
However, in certain operating or failure conditions, for example during flight, it may be desirable to suppress certain forces which act on the control surfaces and thus on the actuator, for example to prevent flutter, shimmy or other unwanted movement of the control system. To damp such forces, a bypass line is typically installed within the hydraulic circuit, linking the extension chamber and retraction chamber of the actuator through a damping orifice. In this damping mode, any movement of the control system, and therefore of the actuator which is connected to it, will cause hydraulic fluid to pass from one of the extension and retraction chamber into the other of the extension and retraction chamber through the damping orifice, thereby damping the movement of the actuator and the control surface.
In some applications, multiple actuators may be used to move a control system. In this situation, it is common to place one actuator in an active mode, i.e. with hydraulic fluid being supplied to both extension and retraction chambers, and the other in the damped mode. Generally, when the damping ratio required is low, the damper is a quadratic damper, which means that should the control system be moved by the active actuator, the damping force generated in the damping device is proportional to the square of the actuator speed and moderate drag is generated at high speed. If the damping ratio required is high, then the drag forces generated by damped actuator at high speed will be very high, which means that the adjacent active actuator may need to be oversized to counteract this damping force. In some applications, therefore, a linear, rather than a quadratic, damping device may be used as this will reduce the damping force for a given velocity, meaning that a smaller actuator may be used.